docs: def top_parents(papers, topn=20, verbose=False):
      """
      Returns a list of :class:`.Paper` that cite the topn most cited papers.
  
      Parameters
      ----------
      papers : list
          A list of :class:`.Paper` objects.
      topn : int or float {0.-1.}
          Number (int) or percentage (float) of top-cited papers.
      verbose : bool
          If True, prints status messages.
  
      Returns
      -------
      papers : list
          A list of :class:`.Paper` objects.
      top : list
          A list of 'ayjid' keys for the topn most cited papers.
      counts : dict
          Citation counts for all papers cited by papers.
      """
  
      if verbose:
          print "Getting parents of top "+unicode(topn)+" most cited papers."
  
      top, counts = top_cited(papers, topn=topn, verbose=verbose)
      papers = [ P for P in papers if P['citations'] is not None ]
      parents = [ P for P in papers if len(
                      set([ c['ayjid'] for c in P['citations'] ]) &
                      set(top) ) > 0 ]
  
      if verbose:
          print "Found " + unicode(len(parents)) + " parents."
  
      return parents, top, counts